PROGESTERONE STIMULATION OF HMG-COA REDUCTASE-ACTIVITY IN CULTURED-CELLS

In a previous study we showed that progesterone (PG) stimulated HMG-Co A reductase (HMGR) activity in rat intestinal epithelial cells (IEC-6) incubated in the presence or absence of low-density lipoprotein (LDL) [1,2]. In the present study we examined further the mechanism of this stimulation. We observed that the stimulation of HMGR activity by PG was completely prevented by cycloheximide. Turnover studies utilizing immunoprecipitation of HMGR-labeled with [S-35]methionine revealed tha t PG increased reductase activity by inhibiting HMGR degradation witho ut affecting the synthesis of HMGR. The stimulation of HMGR activity b y progesterone could be accounted for by a continuous synthesis of HMG R while its degradation was retarded. In the presence of LDL, the acti vity of HMGR in IEC-6 cells was effectively inhibited, however PG was able to stimulate HMGR in the presence of LDL. This effect was not due to an interference of normal cellular metabolism of LDL, since PG had no effect on the cellular uptake and lysosomal degradation of I-125-L DL. PG did not affect of the rate of lysosomal hydrolysis of [H-3]chol esteryl linoleate-LDL. The free [H-3]cholesterol derived from [H-3]cho lesteryl linoleate-LDL moved to the cell membrane and effluxed to HDL( 3) in the medium at the same rate in the presence or absence of PG. Al though PG did not affect LDL metabolism, pre-treatment of cells with L DL delayed the onset of HMGR stimulation by PG. In IEC-6 cells deprive d of LDL for 24 h, the HMGR activity was stimulated immediately follow ing PG addition. In cells pre-treated with LDL for 24 h. the stimulati on was delayed by 4 h. Treatment of cells with 25-hydroxycholesterol c ompletely prevented PG stimulation of HMGR activity. We propose that t he stimulation of HMGR activity in the presence or absence of LDL is r elated to the ability of PG to attenuate the formation and/or action o f intracellular HMGR repressor molecules which accelerate the degradat ion of HMGR.